The GaN—AlN material system is well suited for UV optoelectronic devices, such as UV LEDs and UV Lasers, because its energy gap can be tuned by changing its alloy composition to cover all three regions of the UV electromagnetic spectrum: UV-A (340-400 nm), UV-B (290-340 nm) and UV-C (200-290 nm) [1-4]. Such semiconductor devices are expected to be lightweight, compact and have low power requirements. In addition, nitride semiconductors are physically robust, chemically inert, have high corrosion resistance and are non-toxic. These properties also make them attractive for use in hostile environments and at high temperatures. UV LEDs in particular are crucial for a number of applications such as UV photolithography, water/air purification, surface disinfection, free-space non-line-of-sight communication, epoxy curing, counterfeit detection and fluorescence identification of biological/chemical agents.
Despite intense efforts worldwide, the external quantum efficiency (EQE) of the majority of AlGaN based UV LEDs reported in the literature is generally 1-5% [5-12] and there is only a recent report of about 10% efficiency [9]. This is to be contrasted with blue LEDs based on InGaN alloys, whose EQE is as high as 60%. The EQE is defined as the product of the internal quantum efficiency (IQE), injection efficiency (IE) and extraction efficiency (EE). Thus, the poor EQE of deep UV-LEDs may be the result of poor IQE, IE, EE or a combination of all three factors. The IQE is the ratio of electron-hole pairs, which recombine radiatively, to those which recombine though all recombination routes (including radiative recombination). The IQE depends sensitively on extended and point defects, which act as non-radiative recombination centers. The IE is the ratio of electron-hole pairs arriving and recombining in the active region of the device to those injected in the n- and p-contacts from the power source. The EE is the ratio of photons extracted in the free space to those generated inside the device.
Currently, the majority of AlGaN-based deep UV-LEDs are grown on (0001) sapphire substrates [7-12, 24, 69, 70]. FIG. 1 shows the schematic of a typical deep UV LED grown on a sapphire substrate according to the prior art. For efficient heat sinking, the LEDs are generally flip-chip bonded with the p-side down onto appropriate heat conducting submount and the light is extracted from the sapphire side. The limitations of this LED structure are discussed below.
Due to the high lattice mismatch between AlGaN and sapphire (˜14%), such devices suffer from high concentrations of threading defects, which lower their IQE [16]. Besides the high concentration of the threading defects, another source leading to poor IQE is the incorporation of oxygen in AlGaN. Oxygen impurities incorporate readily during growth of AlGaN films due to the high chemical affinity of aluminum for oxygen [14, 15]. Oxygen is known to introduce states in the center of the energy gap in AlGaN alloys with high AlN mole fraction, which are likely to be potent recombination centers.
The IE of UV LEDs is limited by the relatively poor doping efficiency of the p-AlGaN (low hole density) and the low diffusion length of the injected holes. Furthermore, careful design of the electron-blocking layer (EBL) is required to prevent electrons from escaping the active region and also to allow holes to reach the active region. In this respect, it should be pointed out that in devices such as the one shown in FIG. 1, the spontaneous-polarization and piezo-polarization produce an electric field in the electron blocking layer (EBL) and in the barriers of the quantum wells (QWs) that oppose hole injection into the active region.
One factor limiting the EE in UV LEDs on sapphire is absorption in the p-type layers, typically p-GaN. One approach to mitigate this absorption loss is by replacing the p-GaN with transparent p-AlGaN [28, 29]. This scheme was shown to give a 2× increase in output power [28]. In addition to the aforementioned absorption losses in the p-side, the EE in current UV LEDs is limited by losses associated with wave-guiding in the AlN buffer and the sapphire substrate as well as losses due to Snell's law as the light travels from the high index of refraction n-AlGaN sequentially to AlN buffer/sapphire/air. A potential solution to the poor EE is the removal of the sapphire substrate as well as the AlN buffer and texturing the n-AlGaN. However, such a process, while commonly practiced in blue LEDs using a laser lift-off process, is difficult in deep UV LEDs because they employ AlN buffer instead of GaN buffer. The AlN buffer does not absorb enough light from the processing laser to undergo thermal decomposition as the defective GaN buffer does in blue LEDs. In addition, such steps, even if they are possible, add to the cost of the devices.
There are a few references reporting an inverted device structure, based on InGaN QWs, emitting in the blue region of the spectrum, in which the p-GaN is deposited first on the sapphire substrates [37-39]. There are also a few reports of UV LEDs, grown on n-SiC [13]. However, it should be stressed that in these devices the SiC is not part of the device structure, and it only acts as a substrate for the epitaxial growth.